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In a study on the use of mineral slags as substitutes for silica sand in abrasive blasting, lung tumors were unexpectedly found in rats treated with quartz (14808607) as positive fibrogenic dust controls. Groups of 85 male Fischer-344-rats were treated with single 20 milligram doses of test material by intratracheal instillation; a vehicle control group (filtered, deionized water) was included. The quartz dusts Min-U-Sil and Novaculite were used as the positive control substances. Interim sacrifices of ten rats per group were performed at 6, 12, and 18 months postexposure, with the terminal sacrifice at 22 months. A lung tumor was first observed at the 12 month sacrifice in a rat injected with Min-U-Sil. In animals alive at the time the first lung tumor was observed, the incidence of tumors in the Min-U-Sil group was 30/67 and that in the Novaculite group was 21/72; only one lung tumor was observed in the vehicle control group. Of the Min-U-Sil group, 25 percent had lung tumors larger than 0.3 centimeters in diameter while only 9.7 percent of Novaculite group had lung tumors that large. This may have been related to the higher surface area and smaller particle size of Min-U-Sil. In none of the animals injected with the mineral slag samples were there any granulomas similar to those induced with the quartz nor were there areas of dense collagen. Preliminary evaluation of tissues from the groups of rats treated with mineral slags indicated tumor incidences similar to that of the vehicle control group, one to two tumors per test group.